The present invention relates to a machine tool with a pneumatic hammer mechanism particularly a rotary cutting chisel or a purely chiseling machine tool.
In a motor-driven pneumatic hammer mechanism a piston-shaped exciter accelerates indirectly, via an air spring, a piston-shaped beater. Seals and other lateral conditions of the hammer mechanism limit the pressure of the air spring to approximately 20 bar, thus the acceleration of the beater depends on its weight and its facial area. A hammer mechanism with a rapidly operating beater requires a light beater with a large facial area. US 2009 133893 A describes a beater showing a radially extending bore in order to reduce its weight with the facial area remaining constant.
The volume of the pneumatic chamber sets high requirements to the tolerances of the production. Any contacting of the exciter and the beater leads the hammer mechanism to fail. During the compression of the air spring the distance of the exciter from the beater reduces to approximately 1 mm, in order to generate the pressure of 20 bar necessary for the required beating performance. The high pressure leads to a local deformation of the hammer mechanism, for example the sealing ring surrounding the beater shifting and deforming. By the mechanical and thermal constant stress of the elastic sealing ring it tends to age, making it less resistant to shifting. Even a minor shifting may then lead to a sufficient displacement of the volume of the air spring with a subsequent punctual contacting of the exciter and the beater.